Wireless networks have become increasingly popular, as computers and other devices can be coupled for data communications without requiring wired connections between the network nodes. One set of standards for wireless networks is the IEEE 802.11 standards, but other wireless standards or protocols might be used instead. Because wireless networks are expected to operate in unfavorable conditions, such as in the presence of reflections, interference, movement of receivers/transmitters, etc., much effort is needed to correctly transmit and receive data over a wireless channel.
A typical node in a wireless network (referred to in the standards as a “station”) includes a receive chain and a transmit chain. A transmit chain typically includes some digital processing and analog circuitry (RF, baseband, etc.) that causes a signal to be transmitted into the wireless channel. A receive chain typically includes one or more antenna, RF circuitry and other analog circuitry, and digital processing that seeks to output a data stream that represents what the sending transmit chain received as its input and transmitted into the wireless network. Of course, where there are unrecoverable errors, there is a mismatch between what the sending transmit chain received and what the receiving receive chain outputs. In some cases, a receiver uses multiple antennas to improve reception of the signal and/or reduce errors from a sending transmit chain.
Because of the expected conditions, the receive chain includes various components designed to ensure that signals can be largely recovered correctly. Several techniques have been in use to recover signals. One technique is the use of MIMO (multiple-input, multiple-output) channels. MIMO is also used to increase bandwidth for given conditions and/or to improve reception without increased bandwidth.
In the IEEE 802.11 standards, there are at least two widely-used standards, 802.11a and 802.11b, and communication systems and devices might be required to support both standards and/or be required to operate in areas where both are being used. Enhancements to the 802.11 standards have been in place, such as the 802.11g standard that allows for OFDM transmissions (802.11a is an OFDM transmission protocol) in the 2.4 GHz band (the band used for 802.11b).
The 802.11a protocol supports OFDM transmissions in the 5 GHz band for data rates of 6 to 54 million bits per second (“Mbps”). The 802.11b protocol supports DSSS transmissions in the 2.4 GHz band for data rates of 1, 2, 5.5 and 11 Mbps. The 802.11g protocol mixes OFDM and DSSS protocols in the 2.4 GHz band for data rates of 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48 and 54 Mbps. Data transmissions are well known for these protocols, so they need not be set forth herein. They are described, for example, in ANSI/IEEE Std 802.11, 1999 Edition; IEEE Std 802.11b, 1999; IEEE Std 802.11a, 1999/Amd 1:2000(E), IEEE 802.11g-2003 Amendment to IEEE Std 802.11, 1999. Those references are incorporated by reference herein for all purposes.
While 802.11a, 802.11b and the like standard wireless protocols are in use, even higher performances are now desired. The possible use of new protocols is complicated by the fact that newer protocol devices might have to coexist with legacy devices and should preferably not cause undesirable errors in legacy devices by their presence. In order to avoid conflicts, extended devices that extend beyond the limits of the standard and legacy devices that comply with an existing standard and are not necessarily aware of extended standards need to coexist in a common communication space.
Coexistence is where differing devices can operate in a common space and still perform most of their functions. For example, an extended transmitter transmitting to an extended receiver might coexist with a legacy transmitter transmitting to a legacy receiver and the extended devices can communicate while the legacy devices communicate, or at least where the two domains are such that one defers to the other when the other is communicating. Coexistence is important so that the adoption and/or use of extended devices (i.e., devices that are outside, beyond or noncompliant with one or more standards with which legacy devices adhere and expect other devices to adhere) do not require replacement or disabling of existing infrastructures of legacy devices.
Beamforming is a technique wherein a transmitter adjusts its signal such that the signal is stronger in some directions than in others. Where the overall power dissipation is the same, beamforming causes the signal in the favored directions to increase (providing increased robustness, range, etc.) while the signal in the disfavored directions decreases.